The semiconductor industry demands that every new technology has to deliver increasing performance and density but with lower cost. Flash memory is the mainstream non-volatile memory in today's market. However, Flash memory has a number of limitations that is posing a significant threat to continued advancement of memory technology. Current stand alone and embedded memory technologies suffer many drawbacks due to the extreme demands of scaling dictated by Moore's law.
Aggressive scaling has put the industry in such a predicament that only new materials, devices and newer system architecture will be able to provide the low energy, high speed, performance and high reliability requirements for the future. Therefore, the industry is exploring alternative memories to replace Flash memory. Contenders for future memory technology include magnetic storage random access memory (MRAM), ferroelectric RAM (FeRAM), and resistive switching memories such as phase change RAM (PCRAM), resistive RAM (RRAM), ionic memories including programmable metallization cell (PMC) or conductive bridging random access memory (CBRAM). These memories are also called as emerging memories.
One technology that holds promise to deliver such performance is Conductive Bridging Random Access Memory (CBRAM). Conventional non-volatile memories (NVM) technologies for both discrete and embedded applications require operational conditions that are incompatible with modern low voltage logic CMOS designs. These requirements create complex integration issues as well as costly process and array concept especially for embedded NVM use models. In contrast, CBRAM technology offers simple integration and scalable operational conditions. CBRAM may be integrated into copper and aluminum back end logic CMOS processes with minimal number of added masks with no adverse impact to the CMOS technology. CBRAM offers promising operation parametrics which are increasingly difficult to achieve with other types of memories such as low operational voltage (<1 V), low operational current (1 A), and ultrafast switching (<100 ns). These unique features make CBRAM technology an ideal candidate for embedded applications.
CBRAM technology is also known by other names such as programmable metallization cell (PMC) solid electrolyte memory, nano-ionic resistive memory, electrochemical memory (ECM). CBRAM memory devices utilize solid state electrochemistry to modulate the resistance of certain materials known as solid electrolytes by reversibly creating a nanoscale conductive link inside them when biased by small voltages.
As an example, devices using this technology may be composed of a thin film of silver doped chalcogenide or oxide glass sandwiched between a silver anode and an inert cathode. Under the influence of an electric field the electron current from the cathode reduces an equivalent number of Ag-ions as injected from the anode and a metal-rich electrodeposit is thereby formed in the electrolyte. The magnitude and duration of the ion current determines the amount of Ag deposited and hence the conductivity of the pathway. The electrodeposit is electrically neutral and stable; however, the formation process can be reversed by applying a bias with opposite polarity. The reverse ion current flows until the previously injected Ag has been oxidized (Ag→Ag++e) and deposited back to the electrode which supplied the metal. Thus, the resistivity increases again until the high value of the solid electrolyte is reached. This resistive switching of the material caused by the formation and removal of the metallic Ag pathway can be used for information storage. The basic storage element consists of an access transistor and a programmable resistor (1T-1R) (similar to the DRAM one transistor and one capacitor cell).